Ion (or charge) accumulation devices are well known in the art and can take many forms such as, for example, three-dimensional ion traps and two-dimensional (or “linear”) ion traps. FIG. 1 illustrates an example of a three-dimensional ion trap 100. This type of ion trap may be constructed from electrodes formed by hyperboloids of revolution forming a top hyperbolic shape 102 and a bottom hyperbolic shape 104 (also termed end caps), and a center or ring electrode 106 that is also a hyperboloid of revolution. An alternating voltage may be applied to the center electrode 106 to form a three-dimensional quadrupolar restoring force directed towards the center of the electrode assembly. Ions are confined within an electrodynamic quadrupole field when their trajectories are bounded in the (r) and (z) directions. One or both end caps 102 and 104 may have one or more apertures 108 and 110. One of these apertures 108 or 110 is typically utilized to introduce externally produced ions into the ion trap 100, or alternatively to introduce an electron or photonic beam in the case of in-trap ionization. One or both of the apertures 108 and 110 may also be utilized to eject ions from the ion trap 100 in the (z) directions during the course of known ion processing techniques, for example analytical scans in the case of mass spectrometry. An ion detector (not shown) may be positioned to receive ions ejected from at least one of the apertures 108 or 110 to measure ion flux, count the number of ions received, etc.
FIG. 2 illustrates an example of a linear ion trap 200. This type of ion trap may be formed from four electrodes 202, 204, 206 and 208 of hyperbolic cross-section arranged about a central longitudinal axis, designated in FIG. 2 as the z-axis. These electrodes 202, 204, 206 and 208 may be provided in the form of cylindrical rods to approximate the hyperbolic shapes, as in the example illustrated in FIG. 2. Typically, one opposing pair of electrodes 202 and 204 are electrically interconnected, as are another opposing pair of electrodes 206 and 208. An alternating voltage is applied between the rod pairs 202/204 and 206/208. The alternating electric field thus generated creates a two-dimensional restoring force on an ion, which is directed towards the center axis of the rod structure. The quadrupolar restoring field is equivalent to a trapping field that traps the ions in the direction transverse to the central axis. If plates 212 and 214 are located at the ends of the rod structures and have a DC voltage applied to them, a force will be applied to an ion that is directed along the axis of the rods 202, 204, 206 and 208. Thus, ions will be confined along the x-axis and y-axis directions due to the alternating voltage gradient, and along the z-axis by means of the DC potential applied to the end plates 212 and 214. Typically, ions are introduced axially into this type of ion trap 200 through an aperture of a plate 212 or 214. Ions may be ejected axially or, alternatively, radially between adjacent rods 202, 204, 206 and 208 or through apertures or elongated slots formed in one or more of the rods 202, 204, 206 and 208. Other types of linear ion traps can be formed from utilizing more than four electrodes 202, 204, 206 and 208, such as six or eight, which will form higher order multipole fields besides quadrupole such as hexapole or octopole as is well known in the art. Additionally, multipole electrode sets may be operated as mass filters, collision cells, or simply ion guiding or focusing devices, as is also well-known.
It is known in the art to selectively eliminate ions of a specified mass-to-charge ratio from ion accumulation devices. In an ion trap, for example, selected ions may be eliminated (ejected) by applying a supplemental alternating voltage to the pair of end caps in the case of a three-dimensional ion trap or a pair of opposing rods in the case of a linear ion trap. Ions with a mass-to-charge ratio having a natural (or secular) frequency of oscillation matching the frequency of the supplemental voltage will be ejected from the trap in the direction of the applied supplemental field. Waveforms comprising multiple frequencies may be used to eject ions with multiple mass-to-charge ratios. If these multiple frequencies are applied during the time that ions are entering the ion accumulation device, unwanted ions can be continuously removed as they enter. The development of space charge in an ion accumulation device is undesirable for a number of reasons. For example, large amounts of space charge can result in a shift in the ion frequencies such that they are no longer in optimal resonance with the supplemental frequencies. In a similar manner, ions that are close in frequency to a supplemental frequency can be shifted into resonance with that frequency and thereby be ejected. Therefore, a well recognized need exists for addressing space charge in the design and operation of ion accumulation devices.
In methods such as disclosed in U.S. Pat. No. 6,987,261, the number of charges in an ion accumulation device or ion trap mass spectrometer is based on allowing the charge flux to change and to control the time period during which charges are accumulated. This type of technique may be explained by referring to FIGS. 3A and 3B of the present disclosure. As the charge flux increases because the sample amount increases (FIG. 3A), the ion accumulation time is reduced so as to accumulate a constant number of charges (FIG. 3B). Therefore, at a low sample amount the ion accumulation time is large (Δta1). As the sample amount increases, the ion accumulation time becomes smaller (Δta2). The charges are introduced into the ion accumulation device in a single packet of variable length due to the variable accumulation time. As the period decreases the length of the ion packet decreases, but the charge density actually increases. Therefore, increasing the sample amount will cause an undesirable increase in the ion space charge density, which results in the undesired shift of the ion frequency.
Accordingly, a need continues to exist for more effective apparatus and methods for reducing the undesired affects of space charge in an ion trap or other device employed for charge accumulation. In accordance with certain implementations taught in the present disclosure, such a need may be met by controlling the ionic charge flux entering the accumulation device in a fixed accumulation time period, Tac, rather than varying the accumulation time period, and thereby maintaining the space charge density. An additional benefit provided by certain implementations taught in the present disclosure is to maintain a constant scan-to-scan time because the ion accumulation time Tac is kept constant, while the charge flux is modulated. This is in contrast to the prior art in which the accumulation time is varied as the charge flux from the ion source changes.